The use of membranes for separation of mixtures of liquids and gases is well-developed and commercially very important art. Such membranes are traditionally composed of a homogeneous, usually polymeric composition through which the components to be separated from the mixture are able to travel at different rates under a given set of driving force conditions, e.g. trans-membrane pressure and concentration gradients. Examples are the desalination of water by reverse osmosis, separation of water/ethanol mixtures by pervaporation, separation of hydrogen from refinery and petrochemical process streams, enrichment of oxygen or nitrogen from air, and removal of carbon dioxide from natural gas streams. In each separation, the membranes must withstand the conditions of the application, and must provide adequate flux and selectivity to be economically attractive.
One type of membrane that may be used to separate oxygen from non-oxygen gases or hydrogen from non-hydrogen gases is made of a solid electrolyte material. A solid electrolyte is an inorganic crystalline material that, while being impermeable to gases, has the property of conducting oxygen ions (O2−) or protons (H+) through vacancies in its crystalline structure. In order to maintain electric charge neutrality, certain solid electrolyte membranes must include a separate electron-conductive path. Other solid electrolyte membranes are made of materials that, at elevated temperatures, can simultaneously conduct oxygen ions and electrons or simultaneously conduct protons and electrons. Examples of these oxygen ion conductive materials include certain perovskites such as LaxSr1-xCo03-y, LaxSr1-xFeO3-y and LaxSr1-xFeyCo1-yO3-z are examples of mixed conductors. One example of a proton conductive material is a cermet, a composite of metal and sintered ceramic. Other examples of proton conductive materials include the single-phase mixed metal oxide materials of the formula: AB1-xB′xO3-y wherein A is selected from Ca, Sr or Ba ions, B is selected from Ce, Zr, Ti, Tb, Pr, or Th ions, B′ is selected from Yb, In, Ru, Nd, Sc, Y, Eu, Ca, La, Sm, Ho, Tm, Gd, Er, Zr, Gb, Rh, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Al, Ga, or In ions (or combinations thereof), x is greater than or equal to 0.02 and less than or equal to 0.5, and y is such that the electrical neutrality of the crystal lattice is preserved. These oxygen ion or proton conductive membranes are often called mixed oxide conducting membranes.
Other terms used to describe these membranes include mixed ion and electron(ic) conducting membranes, mixed proton and electron(ic) conducting membranes, ion transport membranes, oxygen transport membranes, hydrogen transport membranes, solid state membranes, mixed conducting metallic oxide, and mixed conducting multicomponent metallic oxide membranes. Regardless of the name utilized, these materials have the ability to transport oxygen ions (O2−) or protons (H+) through their crystalline structure.
Using oxygen conductive mixed oxide conducting membranes as an example, at elevated temperatures, the mixed oxide conducting material contains mobile oxygen ion vacancies that provide conduction sites for transport of oxygen ions through the material. The membrane is in part driven by a difference in oxygen partial pressure across the membrane. When the surface of the membrane is exposed to the relatively higher O2 partial pressure gaseous atmosphere, the molecular oxygen in the gaseous atmosphere adjacent the surface reacts with electrons and the oxygen vacancies in the crystalline structure of the material to product oxygen ions O2−. The oxygen anions diffuse through the mixed conductor material to the opposite surface of the membrane which is exposed to the relatively lower O2 partial pressure. At the opposite surface, the oxygen anions give up their electrons and form molecular oxygen. The molecular oxygen then diffuses into the gaseous atmosphere adjacent the surface of the membrane exposed to the lower O2 partial pressure gaseous atmosphere. These materials transport oxygen ions selectively, and assuming a defect-free membrane and lack of interconnecting pores, they can act as a membrane with an infinite selectivity for oxygen.
Proton conductive mixed oxide conducting membranes operate in much the same way and are similarly in part driven by a difference in hydrogen partial pressure across the membrane. When the surface of the membrane is exposed to the relatively higher H2 partial pressure gaseous atmosphere, hydrogen molecules disassociate into protons and electrons which migrate through the membrane to the opposite surface where they recombine into hydrogen molecules. The thus-formed hydrogen molecules then diffuse into the gaseous atmosphere adjacent the membrane surface. Similar to oxygen conducting mixed oxide conducting membranes, these proton conducting membranes offer the possibility of infinite selectivity for hydrogen.
Mixed oxide conducting membranes have been successfully made in flat or planar shapes and large cylindrical tubes (with outer diameters of greater than 1 cm), but have had limited commercial success because of their relatively low surface area compared to small-diameter (for example, an outer diameter/inner diameter of 670/490 microns) hollow fibers.
Large cylindrical tubes may be distinguished from small-diameter hollow fibers not only on the basis of size, but also in their manner of manufacture. Larger diameter tubes are typically made by extrusion through a die of, or cast from, a composition having a very high solids content and very low solvent/dispersant content. This is done in order to provide the rigidity necessary for such structures to avoid collapse after being extruded or being removed from the cast.
As an example of small-diameter hollow fibers, US 20090169884 discloses that they may be made by injection of a relatively high solvent content suspension (e.g. 10-33% or even 20-25% by weight of the suspension) through an annulus of a hollow fiber spinnerette and injection of a bore fluid from a bore on the inside of the spinnerette annulus. Preferred polymers are copolymers having both soft and hard segments. The nascent fiber is passed through a short air gap and directly into a coagulating fluid to facilitate phase change of the polymeric binder in the suspension. The coagulated fiber exhibits excellent processability and may be drawn and wound on a take-up roll, drum, spool, or bobbin.
During the production of mixed oxide conducting membranes, the unsintered, or “green”, plate, tube, or fiber is subjected to a heat treatment to pyrolize the binder and sinter the ceramic material to yield a dense, monolithic structure of very low porosity. These fibers are variously described as non-porous or micro-porous. The degree of porosity may vary from across one surface of the membrane to the other, but the pores do not interconnect.
Mixed oxide conducting membranes can be placed in two major groups: those that conduct oxygen ions and those that conduct protons. Membranes from the first group may be used to separate oxygen from gas mixtures containing oxygen. Membranes from the first group may also be used to produce oxygen for reaction with a feedstock, for example, light hydrocarbons such as methane, natural gas, or ethane. Membranes from the second group are typically used to separate hydrogen from gas mixtures containing hydrogen, such as syngas. Regardless of whether they conduct oxygen ions or protons, mixed oxide conducting membranes appear to be well suited for oxygen or hydrogen production or separations since they can be operated in a pressure driven mode. Thus, production of oxygen (or production of the reaction product of oxygen and light feedstock) or hydrogen is driven by the difference in the partial pressure of the gas of interest between the two sides of the membrane.
Mixed oxide conducting membranes must exhibit a variety of mechanical and physical properties in order to withstand handling during processing into gas separation modules and also provide a desirable operational lifetime while maintaining a suitable level of performance under operating conditions. More particularly, they should have sufficient strength to resist cracking during formation of the eventual gas separation module from the component fibers. Fibers of especially small diameters are especially susceptible to breaking during handling. They should also be capable of withstanding elevated pressure differentials and elevated oxygen, hydrogen, carbon monoxide, carbon dioxide, moisture, or other chemical conditions without significant loss in its oxygen ion or proton transport ability. However, mixed oxide conducting materials exhibit different degrees of these mechanical and physical properties. For example, many mixed oxide conducting materials exhibit excessive creep at elevated temperature which may cause the membrane to deform and ultimately crack under certain pressures.
Permeation of oxygen across a mixed oxide conducting membrane may be modeled upon the Wagner theory:
            J              O        ⁢                                  ⁢        2              =                  RT                  4          ⁢                                          ⁢                      tn            2                    ⁢                      F            2                              ⁢                                    σ            e                    ⁢                      σ            I                                                σ            e                    +                      σ            I                              ⁢              Ln        ⁡                  (                                    P                              1                ,                                  O                  ⁢                                                                          ⁢                  2                                                      /                          P                              2                ,                                  O                  ⁢                                                                          ⁢                  2                                                              )                                        Since        ⁢                                  ⁢                  σ          e                    >>      σ|        ,                                        σ            e                    ⁢                      σ            I                                                σ            e                    +                      σ            I                              =              σ        I                        J              O        ⁢                                  ⁢        2              =                                                      σ              I                        ⁢            RT                                4            ⁢                                                  ⁢                          tn              2                        ⁢                          F              2                                      ⁢                  Ln          ⁡                      (                                          P                                  1                  ,                                      O                    ⁢                                                                                  ⁢                    2                                                              /                              P                                  2                  ,                                      O                    ⁢                                                                                  ⁢                    2                                                                        )                              ⁢                          ∝                                                  σ              I                        ⁢            T                    t                ⁢                  Ln          ⁡                      (                                          P                                  1                  ,                                      O                    ⁢                                                                                  ⁢                    2                                                              /                              P                                  2                  ,                                      O                    ⁢                                                                                  ⁢                    2                                                                        )                              where JO2 is the oxygen flux defined as flow rate per unit area, σe and σl are intrinsic electronic and ionic conductivity of the material, R is the ideal gas constant, T is the absolute temperature, t is the membrane thickness, n is the charge on the charge carrier (which in the case of oxygen ions is 2), F is Faraday's constant, P1,O2 is the oxygen partial pressure at the feed side and P2,O2 is the oxygen partial pressure on the permeate side. The permeation of hydrogen across a mixed oxide conducting membrane may be derived from the above equations with appropriate substitution of n and the partial pressures of hydrogen on the opposite sides of the membrane.
As seen in the above derivation, flux is directly proportional to ion conductivity, operating temperature and pressure ratio while it is inversely proportional to membrane thickness. Thus thinner films could lead to higher oxygen fluxes, reduced surface areas, lower operating temperatures, and smaller oxygen partial pressure differentials across the mixed conductor material.
Although mixed oxide conducting membranes present the possibility for infinite selectivity, there is a tradeoff between flux and mechanical strength. As the thickness of the mixed oxide conducting material is decreased, the mechanical strength correspondingly decreases. Because a mixed oxide conducting membrane must possess a minimum amount of strength to withstand manufacture, handling, and operation (especially in reactive environments), there is a limit to how much the thickness may be decreased. On the other hand, as the thickness is increased in order to provide the necessary mechanical strength, flux suffers.
Wu, et al. fabricated densified oxygen separation membranes from nano size SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) powders synthesized via a flame aerosol synthesis (FAS) method and densified oxygen separation membranes from SCFZ powders synthesized by the traditional solid-state reaction (SSR) method. Z. Wu, X. Dong, W. Jin, Y. Fan, N. Xu, “A Dense Oxygen Separation Membrane Deriving From Nanosized Mixed Conducting Oxide”, Journal of Membrane Science 291 (2007) 172-179. This was done by uniaxially pressing the powders separately into 16 mm disks followed by sintering and polishing to a desired thickness. The crystal structure, morphology, oxygen desorption property and oxygen non-stoichiometry of the monolithic membranes were characterized. Compared with SCFZ synthesized by the SSR method, the densification temperature of SCFZ membranes was reduced and the oxygen permeation flux was increased by 40% at the elevated temperatures (1,073-1,223° K) when SCFZ-FAS powders were used as the starting material. However, because the membranes were produced by polishing sintered disks made from uniaxially pressed FAS powder, a relative great thickness (0.8 mm) resulted.
The thickness of the separation layer thickness can be reduced by increasing the porosity asymmetry of a monolithic fiber. In all-polymeric fibers hollow fibers, a desired degree of asymmetry across the thickness of the fiber wall can be adjusted in a complex manner by varying the content of the bore fluid, the content of the coagulant, the solvent content of the dope formulation, and the residence time of the nascent fiber in the coagulation bath. The degree to which this technique is effective is based upon the baseline asymmetry. However, this approach is limited in application to hollow fibers made of a matrix of polymer and ceramic particles. This is because such composite fibers have a baseline degree of porosity asymmetry that is relatively lower than that of all-polymer fibers due to their lower solvent content. Thus, it is difficult to control the thickness of the densified layer. As a result, the thickness (and resultant overall flux) of the separation layer is difficult to precisely controlled.
The thickness of the separation layer can also be reduced by using a two-layered composite fiber produced by the spin dope technique such as that disclosed in U.S. patent application Ser. No. 13/174,682 filed on Jun. 30, 2011. In that method, the spinnerette is modified to form a thin sheath and a thick core. A core has an interconnecting network of pores while the sheath is gas-tight. These differing morphologies are produced by using a first ceramic compound with a higher melting point in the core and a second ceramic compound with a lower melting point in the sheath. The composite fiber is sintered under conditions sufficient to densify the sheath while retaining the interconnecting network of pores in the core. However, this approach requires the ceramic compound in the core dope to have a higher melting point than that in the sheath dope.
The thickness of the separation layer can also be reduced by using a two-layered composite fiber produced by the spin dope technique such as that disclosed in U.S. patent application Ser. No. 13/194,990, entitled “HOLLOW CERAMIC FIBERS, PRECURSORS FOR MANUFACTURE THEREOF UTILIZING NANOPARTICLES, METHODS OF MAKING THE SAME, AND METHODS OF USING THE SAME”, and filed concurrently herewith. In that method, the spinnerette continuously extrudes a thin sheath and a thick core. The nascent fiber is coagulated, dried, and sintered. The resultant core has an interconnecting network of pores while the sheath is gas-tight. These differing morphologies are produced by using a relatively small particle size ceramic material in the unsintered sheath as compared to the ceramic material of the core. The relatively small particle size depresses the melting temperature of the ceramic material of the sheath (as compared to the material in bulk form). This allows the sheath to be sintered and densified to gas-tightness while the core is not fully sintered and remains porous and non-gas-tight. However, this approach requires the ceramic material in the sheath dope to have a smaller particle size than that of ceramic material the core dope.
Thus, it is an object of the invention to provide a solid state electrolyte membrane that exhibits both satisfactory flux and mechanical strength. It is a further object of the invention to provide greater control over the thickness of the separation layer. It is a still further object of the invention to provide greater freedom in selecting the ceramic compound for use in the separation layer without regard to melting temperature differences or particle size differences.